Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased, and, among these secondary batteries, lithium secondary batteries having high energy density, high operating potential, long cycle life, and low self-discharging rate have been commercialized and widely used.
A lithium secondary battery is a secondary battery which is generally composed of a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator, and an electrolyte and is charged and discharged by intercalation-decalation of lithium ions. Since the lithium secondary battery is advantageous in that it has large electromotive force as well as high energy density and may exhibit high capacity, the lithium secondary battery has been applied to various fields.
A metal oxide, such as LiCoO2, LiMnO2, LiMn2O4, or LiCrO2, is being used as the positive electrode active material constituting the positive electrode of the lithium secondary battery, and metallic lithium, a carbon-based material, such as graphite or activated carbon, or a material, such as silicon oxide (SiOx), is being used as the negative electrode active material constituting the negative electrode. Among these negative electrode active materials, the metallic lithium has initially been mainly used, but, recently, the carbon-based material has been mainly used, because a phenomenon occurs in which the battery is destroyed by damaging a separator caused by the growth of lithium atoms on the surface of the metallic lithium as charge and discharge cycles proceed.
In currently commercially available lithium secondary batteries, a graphite-based active material having high capacity and long life characteristics has been mainly used in the negative electrode. However, the graphite-based active material has a limitation in that, when propylene carbonate (PC) is mixed and used with an electrolyte solution for the purpose of improving low-temperature performance, the propylene carbonate destroys the graphite-based active material by exfoliating layers of graphite of the graphite-based active material.
Thus, in order to address the limitation that the propylene carbonate destroys the graphite-based active material by exfoliating the layers of the graphite of the graphite-based active material as described above, the propylene carbonate is not used with the graphite-based active material, particularly, artificial graphite in which an amount of a functional group or surface defects is small, or a method of performing a high-temperature activation process has been used if there is a need to use the propylene carbonate inevitably.
However, in a case in which binary/ternary electrolytes based on ethylene carbonate (EC) are used as the electrolyte solution without the addition of the propylene carbonate, an operating temperature may be limited due to high melting point of the ethylene carbonate, battery performance may be significantly degraded at low temperature, and the time and cost required for the preparation of the secondary battery may be increased when performing the high-temperature activation process.
Thus, there is a need to develop a new technique which may reduce a side reaction between the propylene carbonate and the graphite-based active material.